1. Field of the Invention
The present invention relates to a pressure sensor including a resistor element, the electric resistance of which is changed in accordance with a strain caused by pressure, stress, or load. In particular, the present invention relates to a pressure sensor for detecting a relatively small pressure of approximately 0 to 2,000 mmH.sub.2 O or a relatively small load of approximately 0 to 100 g with high accuracy.
2. Description of the Related Art
Pressure sensors are widely used for detecting a stress or load applied to components of machines, ships, and vehicles or for detecting an environmental pressure. In this specification, a pressure sensor will also be referred to as a strain sensor or a strain gauge.
A representative strain sensor includes film formed of a resin such as polyester, epoxy, or polyimide acting as a substrate and a thin-film resistor element formed of a Cu--Ni alloy, a Ni--Cr alloy or the like and provided on the substrate by use of deposition or sputtering. Such a strain sensor is used in the following manner. The substrate thereof is adhered on a surface of the member which receives a stress or load to be measured, using an adhesive resin such as cyanoacrylate. The stress or the load are measured by the strain sensor in the following manner. A degree of strain of the member caused by an external force or load is transmitted to the resistor element through the substrate. Since a cross sectional area of the resistor element and a length of an electrical current path are slightly changed by the strain, the electric resistance of the resistor element is changed. The change in the electric resistance is detected as an electric signal in order to measure the degree of the strain, and the stress and the load applied to the member are detected based on the degree of the strain.
Such a strain sensor is used, for example, as a strain sensor for measuring the load of a suspension for a vehicle. The strain gauge is adhered on a surface of a shaft of the suspension with an adhesive resin or the like in order to detect the load applied on a wheel. Such a usage of the strain gauge as for the suspension, namely, the usage under severe conditions of a temperature range of -50.degree. C. to 150.degree. C. and a maximum load of 2 tons for a long period of time has a problem in that the adhering strength of the adhesive resin declines and this causes the strain gauge to be peeled off from the component to be measured.
Researchers including the inventors of the present invention have disclosed a strain sensor for solving the above-mentioned problem in Japanese Patent Application No. 3-282663. This strain sensor includes a pipe-shaped metal substrate, a glass layer formed of a partially devitrified enamel provided on the metal substrate, and a resistor element provided on the glass layer. This strain sensor is incorporated in a shaft of the suspension. The electric resistance of resistor element is changed when the resistor element is strained by stress or load. In the strain sensor, components of the metal substrate and of the glass layer are diffused and mixed with each other at an interface therebetween, and components of the glass layer and the resistor element are diffused and mixed with each other at an interface therebetween. Accordingly, the adhesion between the metal substrate and the glass layer end the adhesion between the glass layer and the resistor element are extremely strong. Since this sensor is of a built-in type, the strain sensor does not peel off from the component even in severe conditions.
Such a strain gauge is used for detecting a load applied on a wheel of a vehicle, namely, a load of approximately 0 to 2 tons. It is difficult to detect a relatively small pressure of approximately 0 to 2,000 mmH.sub.2 O or a relatively small load of approximately 0 to 100 g with such a strain gauge with high accuracy.